1. Field of the Invention
The present invention relates to a liquid crystal display (LCD), and more particularly, to a backlight assembly and LCD having the same capable of preventing optical sheets from wrinkling due to close stacking of optical sheets or prism sheets employed in the backlight assembly.
2. Description of the Related Art
Generally, LCDs are the representative flat panel display. In order to display an image in these LCDs, an electric power is applied to liquid crystal having a specific molecular configuration to thereby vary the molecular arrangement of liquid crystal. The molecular variations of liquid crystal change optical properties such as birefringence, rotary polarization, dichroism and light scattering. The LCDs converts the optical property changes into the variations in the visual properties, thereby displaying a desired image.
LCDs are generally classified into two types according to liquid crystal used: a twisted nematic (TN) type; and a super-twisted nematic (STN) type. These LCDs are also classified into an active matrix display type using switching devices and TN liquid crystal, and a passive matrix display type using STN liquid crystal according to a difference in the driving way.
The two typed LCDs have apparent difference. The active matrix type display is employed in TFT-LCDs utilizing thin film transistors (TFT) as switching element. The passive matrix display does not utilize TFT as switching element. In other words, the passive matrix display does not need complicated circuits such as TFTs.
LCDs are passive device that does not emit light by itself. Thus, LCDs need a backlight assembly as light source to display images. This backlight assembly is established at the rear surface of the LCD panel.
Recently, in order to secure the competitiveness of LCD products, LCDs are being developed in a slimmer and lighter structure. Especially, considering that the LCDs are mainly used for portable computers, the present LCDs deal with lightness more significantly.
In these LCDs, present interests are especially focused on roles and functions of the backlight assembly. This is because size and light efficiency of LCDs become different and mechanical and optical properties are affected depending on the LCD structure to a considerable degree.
There is disclosed a structure of a general backlight assembly used in these LCDs in U.S. Pat. No. 5,467,208.
FIG. 1 is a simplified exploded perspective view of a conventional LCD having the backlight assembly, and FIGS. 2 and 3 are simplified sectional views of the backlight assembly of FIG. 1.
Referring to FIGS. 1 and 2, a backlight assembly includes a lamp 12 for emitting a light, a light guiding plate 20 for guiding the light emitted from the lamp 12, and a lamp cover 14 established at one side of the light guiding plate 20, for surrounding the lamp 12.
As the lamp 12, a cold cathode lamp is mainly used. Light generated from the lamp 12 is incident through a side wall of the light guiding plate 20. Inner surface of the lamp cover 14 is made of a reflector plate. The light guiding plate 20 reflects light generated from the lamp 12, thereby enhancing a light efficiency.
The light guiding plate 20 guides the light generated from the lamp 12 to proceed toward an LCD panel 40 mounted thereon. The light guiding plate 20 includes various patterns such as a fine dot pattern (not shown) formed on the rear surface thereof to convert a light proceeding direction toward the LCD panel 40.
A reflector plate 22 is disposed beneath the light guiding plate 20 and a diffusion sheet 32, a first prism sheet 34, a second prism sheet 36 and a protective film 38 are placed in such an order on the light guiding plate 20.
The reflector plate 22 reflects light beams that are not reflected by the printed patterns of the light guiding plate 20 and leak, toward the light guiding plate 20, thereby reducing loss of incident light to the LCD panel 40 and simultaneously enhancing the uniformity of light beams passing through the light guiding plate 20 upward.
The diffusion sheet 32 is placed between the light guiding plate 20 and the first prism sheet 34 and disperses the light incident from the light guiding plate 20, to thereby prevent a partial concentration of light beams. A plurality of beads 32a and 32b are formed on the front surface and the rear surface of the diffusion sheet 32, and they make uniform the light flux distribution of light beams reflected from the reflector plate 22.
The first prism sheet 34 and the second prism sheet 36 are stacked in the named order on the diffusion sheet 32 and they each have a plurality of triangle prisms at the front surface thereof. The first prism sheet 34 and the second prism sheet 36 collect light beams diffused by the diffusion sheet 32 toward a first and second direction normal to a plane parallel to the LCD panel 40 in order to make a viewing angle narrow, thereby enhancing the brightness at the front side.
The protective film 38 is placed between the second prism sheet 36 and the LCD panel 40 to protect the front surface of the second prism sheet 36. The protective film 38 has a plurality of acryl beads 38a formed at the front surface thereof, to prevent moire phenomenon and rainbow phenomenon. Thus, the light beams generated from the lamp 12 and then passing through multiple optical sheets show images formed by the LCD panel 40.
Meanwhile, when the horizontal viewing angle is important, there is widely used a backlight assembly structure utilizing only either one of the first prism sheet or the second prism sheet having a light collection characteristic and for enhancing the luminance at the front side. In this case, a reflection polarization film is used, which improves a luminance lowering per viewing angles due to the lack of one prism sheet, increases the luminance of the light through the entire surface of the prism sheet and enhances the luminance characteristics every viewing angles.
FIG. 3 shows an example of a backlight assembly having only one prism sheet and a reflection polarization film for improving the brightness characteristic from every viewing angle is also used.
As shown in FIG. 3, a reflection polarization film 39 is placed between the first prism sheet 34 and the protective film 38. The reflection polarization film 39 has smooth front surface in contact with the protective film 38 and smooth rear surface in contact with the first prism sheet 34.
However, the aforementioned conventional backlight assemblies have the following drawbacks. First, since the prisms formed at the front surface of the prism sheet have a shape of ridge, optical sheets wrinkle due to close stacking of another prism sheet or the reflection polarization film.
Second, when a single prism sheet and the reflection polarization film are employed, a contact area is widened between the reflection polarization film and the underlying prism sheet and between the reflection polarization film and the overlying protective film, increasing a friction coefficient. As a result, curl occurs between optical sheets.
Third, since the optical sheets are of compound resin, electrostatic current is easily generated. The electrostatic current allows the optical sheets to be closely attached to each other, however, creating wrinkles between optical sheets.